1. Field of the Invention
The present invention relates to a thin film bulk acoustic resonator, and more particularly to an improved thin film bulk acoustic resonator which effectively suppresses spurious response and which provides excellent frequency characteristics. The present also relates to a method for producing such a thin film bulk acoustic resonator. The present also relates to an improved filter which provides excellent filter characteristics by incorporating such a thin film bulk acoustic resonator. The present also relates to a composite electronic component device comprising such a filter. The present also relates to a communication device comprising such a filter and a composite electronic component device.
2. Description of the Background Art
Component elements to be internalized in an electronic device such as a mobile device are required to be reduced in size and weight. For example, a filter to be used in a mobile device must be small in size, and yet have finely-adjusted frequency characteristics.
As one type of filter satisfying such requirements, filters employing a thin film bulk acoustic resonator (Film Bulk Acoustic Resonator: FBAR) are known.
FIG. 18 is a schematic cross-sectional view showing a conventional thin film bulk acoustic resonator. In FIG. 18, the thin film bulk acoustic resonator 90 is provided upon a substrate 91. The thin film bulk acoustic resonator 90 includes a piezoelectric film 92 and an upper electrode layer 93 and a lower electrode layer 94, in such a manner that the piezoelectric film 92 is interposed between the upper electrode layer 93 and the lower electrode layer 94. A cavity 95 is formed through the substrate 91 so as to expose a lower face of the thin film bulk acoustic resonator 90, this being in order to allow free vibration of the thin film bulk acoustic resonator 90.
When an electric field is applied between the upper electrode layer 93 and the lower electrode layer 94, the electric energy is converted to a mechanical energy due to piezoelectric effects of the piezoelectric film 92. For example, in the case where a piece of aluminum nitride (AlN) having a polarization axis extending in the thickness direction is used for the piezoelectric film 92, the mechanical energy is chiefly converted to vibrations of expansion and compression along the thickness direction. In other words, owing to this mechanical energy, the piezoelectric film 92 expands and compresses in the same direction as that of the electric field.
The equivalent circuit of the thin film bulk acoustic resonator 90 is a circuit which contains a serial resonant circuit and a parallel resonant circuit. Therefore, the thin film bulk acoustic resonator 90 has a resonance frequency as well as an anti-resonance frequency. Given that the thin film bulk acoustic resonator 90 has a thickness t, the thin film bulk acoustic resonator 90 resonates with a resonance frequency fr(=v/λ), which corresponds to a wavelength λ satisfying t=λ/2. Here, v is a sound velocity within the material composing the thin film bulk acoustic resonator 90. Similar to the resonance frequency, the anti-resonance frequency fa is in inverse proportion to the thickness t of the thin film bulk acoustic resonator 90, and is proportional to the sound velocity within the material composing the thin film bulk acoustic resonator 90. In the case of setting the resonance frequency and/or the anti-resonance frequency in a frequency band of several hundred MHz to several GHz, any thin film bulk acoustic resonator 90 supporting such a resonance frequency and/or anti-resonance frequency will have a thickness which allows easy thin film formation at the industrial level. Therefore, in the aforementioned frequency band, the thin film bulk acoustic resonator 90 is useful as a small-sized resonator having a high Q value.
Ideally, the thin film bulk acoustic resonator 90 would only experience vibration in the thickness direction P of the piezoelectric film 92. In practice, however, vibrations along a lateral direction Q may also occur in the thin film bulk acoustic resonator 90, thus resulting in a plurality of lateral propagation modes. These lateral propagation modes are unwanted vibration modes. The lateral propagation modes propagate in a parallel direction to the electrode surfaces, undergo multiple reflections at the side walls of the piezoelectric film 92 or at the ends of the upper electrode layer 93 and the lower electrode layer 94, thus contributing to spurious response. In the case of a device including a plurality of adjoining thin film bulk acoustic resonators, the unwanted vibration modes interfere between adjoining thin film bulk acoustic resonators, and thus the unwanted vibration modes similarly contribute to spurious response. The spurious response ascribable to such lateral propagation modes deteriorates the frequency characteristics of the thin film bulk acoustic resonator.
In order to solve this problem, various techniques have been proposed (see, for example, Japanese Laid-Open Patent No. 2000-31552, and Japanese Laid-Open Patent No. 2000-332568).
FIG. 19A and FIG. 19B are schematic structural diagrams showing a conventional thin film bulk acoustic resonator which is disclosed in Japanese Laid-Open Patent No. 2000-31552. As shown in FIG. 19A, the thin film bulk acoustic resonator includes an acoustic damping material 97a (shown as a region surrounded by dotted lines in the figure), which is provided around a rectangular-shaped electrode 96a (shown as a region surrounded by solid lines in the figure), separately from the electrode 96a and the piezoelectric layer. The acoustic damping material 97a is formed through printing or the like. The acoustic damping material 97a absorbs a substantial amount of lateral-direction acoustic energy, thus alleviating the lateral-direction acoustic energy and suppressing the spurious response. As shown in FIG. 19B, the spurious response can also be suppressed by an acoustic damping material 97b (shown as a region surrounded by dotted lines in the figure) which is provided around an inequilateral rectangular-shaped electrode 96b (shown as a region surrounded by solid lines in the figure) separately from the electrode 96b and the piezoelectric layer. FIG. 19C is a graph showing the passing frequency characteristics in the case where neither acoustic damping material 97a or 97b is provided. FIG. 19D is a graph showing the passing frequency characteristics in the case where the acoustic damping material 97a or 97b is provided. As seen from FIGS. 19C and 19D, the spurious response is suppressed when the acoustic damping material 97a or 97b is provided.
FIG. 19E is a schematic structural diagram showing a conventional thin film bulk acoustic resonator which is disclosed in Japanese Laid-Open Patent No. 2000-332568. The thin film bulk acoustic resonator does not include an acoustic damping material as described above. The thin film bulk acoustic resonator includes an electrode 96c having the shape of an inequilateral non-parallel polygon (i.e., no sides are equal in length, and no sides are parallel) . Due to the use of the electrode 96c having the shape of an inequilateral non-parallel polygon, the thin film bulk acoustic resonator ensures that acoustic waves 98 which originated from a point 900 on a wall are reflected at an opposite wall, thus being prevented from returning to the same point. As a result, the lateral propagation modes are damped, whereby the spurious response is suppressed.
However, in the conventional thin film bulk acoustic resonator disclosed in Japanese Laid-Open Patent No. 2000-31552, it is necessary to additionally provide the acoustic damping material 7, which complicates the production process. Furthermore, the conventional thin film bulk acoustic resonator also has a problem in that not only the unwanted vibration modes but a portion of the desired vibration mode is also damped, thus resulting in deteriorated frequency characteristics.
In the conventional thin film bulk acoustic resonator disclosed in Japanese Laid-Open Patent No. 2000-332568, it is necessary to use an electrode 96c having the shape of an inequilateral non-parallel polygon, which means that the shape of the resonator cannot be freely selected. This problematically reduces the design freedom. Specifically, when the thin film bulk acoustic resonator needs to be subjected to integration, it is difficult to obtain a high degree of integration. The unwanted vibration modes are not completely reflected at the ends of the electrodes but rather are leaked and propagated, thus unfavorably affecting an adjoining resonator.